Precision tool and workpiece positioning apparatus with ringout detection

ABSTRACT

Apparatus for positioning a workpiece and tool in a precise location relative to each other by positioning in a predetermined site the workpiece, and then positioning the tool in a precise predetermined position relative to the site in minimum total time. The apparatus comprises a closed loop position and velocity sensitive servo system connected to the workpiece, the servo system including positioning apparatus for positioning the workpiece in a predetermined site. A position indicator determines the actual position of the workpiece relative to a fixed reference and emits a signal output which is compared, in an error generator, with the desired position of the workpiece relative to the reference. The difference signal, from the error generator, is applied to the positioner and is used to bring the workpiece into the site. The positioner is provided with velocity feedback which cooperates with the positioning signal from the error generator to drive the workpiece into the predetermined site. Thereafter the error between the actual position address and the desired position address, although very small, is fed to second apparatus for positioning the tool a very small amount to precisely position the tool relative to the workpiece. The purpose of this abstract is to enable the public and the Patent Office to determine rapidly the subject matter of the technical disclosure of the application. This abstract is neither intended to define the invention of the application nor is it intended to be limiting as to the scope thereof.

United States Patent [1 1 Hassan et al.

[ 1 PRECISION TOOL AND WORKPIECE POSITIONING APPARATUS WITH RINGOUTDETECTION [75] Inventors: Javathu K. Hassan, Hopewell Junction; Carl V.Rabstejnek, Wappingers Falls; Anthony D. Wutka, Fishkill, all of NY.

International Business Machines Corporation, Armonk, NY.

[22] Filed: Sept. 10, 1973 [21] Appl. No.: 395,502

[73} Assignee:

Primary ExaminerB. Dobeck Attorney, Agent, or Firm-William .1. Dick [57]ABSTRACT Apparatus for positioning a workpiece and tool in a X-TOOLCONTROL 1 cmcun X-SERVO ClRCUlT iuiommc 9' May 27, 1975 precise locationrelative to each other by positioning in a predetermined site theworkpiece, and then positioning the tool in a precise predeterminedposition relative to the site in minimum total time. The apparatuscomprises a closed loop position and velocity sensitive servo systemconnected to the workpiece, the servo system including positioningapparatus for positioning the workpiece in a predetermined site. Aposition indicator determines the actual position of the workpiecerelative to a fixed reference and emits a signal output which iscompared, in an error generator, with the desired position of theworkpiece relative to the reference. The difference signal, from theerror generator, is applied to the positioner and is used to bring theworkpiece into the site. The positioner is provided with velocityfeedback which cooperates with the positioning signal from the errorgenerator to drive the workpiece into the predetermined site. Thereafterthe error between the actual position address and the desired positionaddress, although very small, is fed to second apparatus for positioningthe tool a very small amount to precisely position the tool relative tothe workpiece.

The purpose of this abstract is to enable the public and the PatentOffice to determine rapidly the subject matter of the technicaldisclosure of the application. This abstract is neither intended todefine the invention of the application nor is it intended to belimiting as to the scope thereof.

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FIG. 14

1 PRECISION TOOL AND WORKPIECE POSITIONING APPARATUS WITH RINGOUTDETECTION SUMMARY OF THE INVENTION AND STATE OF THE PRIOR ART Thepresent invention relates to positioning apparatus, and moreparticularly relates to a workpiece and tool positioning system in whichthe workpiece is brought into a predetermined site or small arealocation and thereafter, utilizing the difference between the desiredand actual position of the workpiece, the tool is positioned into aprecise predetermined location relative to the workpiece.

Numerous examples of servo positioning systems exist in the prior art,the servo systems containing means for positioning a load withpositional velocity, or both types of feedback. Both types of feedbackand combinations thereof are advantageous, positional feedbackdecreasing the rate of positioning to inhibit hunting, while velocityfeedback serves to proportionately dampen servo response. An example ofsome of the prior art is the patent to Allen, US. Pat. No. 3,241,015;McKenney, US. Pat. No. 2,913,649; Dick erson, US. Pat. No. 3,377,544;Husted, US. Pat. No. 2,674,708; and Plummer, U.S. Pat. No. 3,660,744.Many of the references above cited utilize both position feedback andvelocity feedback and combinations of both for ultimately positioningthe workpiece. However, when speed of positioning is important, it isdifficult if not impossible to achieve precise location of the workpiecewithout a certain amount of hunting and time for stabilizing (ring out)of the system.

Also described in the art is the provision to position a tool relativeto a workpiece, such as described I C Pattern Exposure by ScanningElectron Beam Apparatus by S. Miyauchi, et al., SOLID STATE TECHNOI;OGY, July 1969, pp. 43-48.

In machine tools, for example, the precise location of the workpiecerelative to the tool which is to perform the operation on the workpieceis extremely important, and if the operation is a repetitive one, thetime for ring out of the system and to stop the hunting of the systemfor operation of the tool on the workpiece can be an extremely importantfactor in system design effecting throughput. The total time for preciseworkpiece to pool positioning is composed of: workpiece Movementacceleration, constant velocity, deacceleration (which is virtuallyeliminated with the apparatus of the present invention) position huntingringout of residual energy, and secondary tool positioning.

In the positioning of, for example, an electron beam sometimes referredto as an E-beam, relative to a semiconductor chip in a semiconductorwafer, to write the circuit pattern onto the topology of the chip, theinitial precise location of the E-beam relative to the chip isabsolutely necessary to achieve the desired result, for subsequentworking of the chip by the E-beam. The system described hereinafter hasfound an advantage in the E-beam technology because of its speed andpreciseness of registration of the tool to the workpiece. Utilizing theelectron beam, as an example, and other examples are given in thespecification, the philosophy of the system and apparatus will be moreeasily understood. Considering a workpiece and tool that must, at leastat the outset, be precisely positioned one relative to the other, if theheavier mass of the two may be positioned within a band of tolerance, orwithin the general locale of the position one relative to the other, thelighter mass or easier to move element, for example, the tool, may bemoved the remaining distance to position the tool in a precise locationrelative to the workpiece. An electron beam, for example, has theinherent capability of compensating for certain errors in the X and Yposition of the workpiece, as long as the error is known. Therefore,starting with this design philosophy, it is not necessary to positionthe XY positioning of the workpiece, such as a semiconductor wafer,within extremely small tolerance bands around an ideal position. It isrequired, however, to resolve the true position of the workpiece withinvery small increments, for example 50 microinches or less. Thus thesystem employed is a position feedback of the tables location. Onceworkpiece location is known and is set within a certain predeterminedtolerance area or site, movement of the workpiece or the XY table whichis used to position the workpiece, may be stopped and the additionalcorrection necessary to precisely position the E-beam may be applied tothe electrostatic deflection plates of the E-beam to position the beamin a precise manner relative to the workpiece. As may be expected,considerable time may be gained because the electromechanical drive ofthe heavier X-Y stage does not have to hunt" for the ideal position. Forexample, when the drive motor connected to the stage (workpiece stage)has been deaccelerated to the approximate location or site specified, itmay be stopped. The position feedback error may then be applied in apredictable manner to electronic compensation to the electrostaticdeflection plates of the E-beam.

The basic velocity and position servo loop, which has already heretoforebeen alluded to, is a standard loop. However, recognizing the value ofan incremental distance to be moved, the intended usage in the systemdescribed is somewhat different. In the system, the loop is tuned toobtain a uniform introduction of energy (acceleration and constantvelocity), and a uniform removal of energy (deacceleration and stop).The intent is to effectively utilize the motors torque to move andarrest the workpiece or X-Y table as quickly as possible with a minimumof structural deformation of the me chanical components. Structuraldeformations which remain after the table is stopped represents energythat must be dissipated as vibrations or ring out. During this unstableperiod no operation may be performed. Accordingly, it is necessary totune the system to achieve an optimum trade-off between the speed ofpositioning and ring out in order to achieve the shortest possible timeuntil work may be performed on the workpiece by the tool.

In view of the above it is a principal object of this system to provideminimum overhead time to position a workpiece relative to a tool and atool relative to a workpiece (tool and workpiece relative to each other)providing maximum throughput of operations of tool on workpiece.

Another principal object of the present invention is to provide aprecise workpiece and tool positioning system in which one of theworkpiece or tool is roughly positioned or positioned within a site, andthen the other of the workpiece and tool is positioned accurately withinthis site and at an exact position of the one relative to the other.

Still another object of the present invention is to provide a controlsystem for element positioning relative to a second element, in whichthe control may be accomplished if desired, by any number of automaticpositioning systems such as employing a digital computer.

Yet another principal object of the present invention is to provide asystem which may be controlled by digi tal information and in which thelow order bits of an actual first element position may be monitored forchanges in status to determine the minimum time when the ring out ofresidual energy is accomplished and for energizing the means fordeflecting or otherwise moving a second element relative to the firstelement.

Other objects and a more complete understanding of the invention may behad by referring to the following specification and claims taken inconjunction with the accompanying drawings in which:

FIG. I is a simplified schematic block diagram of the apparatus of thepresent invention;

FIG. 2 is a composite drawing illustrating the arrangement of FIGS. 2A,2B and 2C which are block diagrams in more detail of the complete systememploying the apparatus of the present invention;

FIG. 3 is a fragmentary side elevational view of a typical X-Y stagewhich may be used with apparatus of the present invention;

FIG. 4 is a fragmentary plan view of a typical tool holder which may beemployed with the apparatus of the present invention;

FIG. 5 is a schematic diagram of a portion of the apparatus illustratedin FIG. 2.

FIG. 6 is a schematic diagram of a typical comparing means which may beemployed in the apparatus of the present invention;

FIG. 7 is a schematic drawing of a portion of the comparing meansillustrated in FIG. 6;

FIG. 8 is a schematic drawing of another portion of the comparing meansillustrated in FIG. 6;

FIG. 9 is a schematic drawing of another portion of the comparing meansillustrated in FIG. 6',

FIG. 10 is a schematic drawing of a latch stop cir cuitry which may beemployed in the apparatus of the present invention;

FIG. II is a fragmentary schematic diagram ofa portion of the automaticmanual switching gate utilized in the apparatus of applicants invention;

FIG. 12 is a schematic diagram of the ring out detection means utilizedto facilitate the operation of the apparatus of the present invention;

FIG. 13 is a schematic block diagram of the circuitry for controllingthe tool;

FIG. 14 is another fragmentary schematic view of a modified tool andcontrol circuit therefor for driving electrostatic deflection plates;and

FIG. 15 is a fragmentary schematic view illustrating in block diagramanother control system which may be employed with the apparatus of thepresent invention.

GENERAL DESCRIPTION Referring now to the drawings and specifically FIG.I thereof, a simplified functional block diagram of apparatusconstructed in accordance with the present invention is illustratedtherein. As shown, a first element or tool 10 is located adjacent asecond element or workpiece W mounted on an X-Y stage 11. Stage drivemeans including motors l2 and I3 are connected respectively for drivingthe X and Y portions of the X-Y stage 11, the motors being responsive toa desired position address from a control unit 100. As illustrated, thedesired address of the position of the workpiece W, and thus the stageII in the X direction is controlled by a signal output along line 10I tothe X servo circuitry 20, while the Y direction of the XY stage iscontrolled by an address along line 102, emanating from the control 100,to the Y servo circuitry 103. As shown, actual positional feedbackinformation is fed back to the X and Y servo circuitry through feedbackloops 20A and 103A respectively. In a like manner, velocity feedback,within the stage drive means, is fed back respectively through lines 12Aand 13A.

The circuitry in both the X and Y servo circuitry 20 and I03 is set sothat when the X-Y stage II and thus the workpiece W is within a presetpredetermined tolerance, or within a small site, (that is the addressdifference between the desired position and the actual position iswithin a very small tolerance) the motors l2 and 13 are latched.Thereafter, the address difference which still exists, albeit verysmall, is fed to an X-tool control circuitry and Y-tool controlcircuitry I04 to effect a minor correction to the tool 10, causing thetool 10 to be realigned relative to the workpiece.

The System Because the Y servo circuitry 103, associated motor and Ytool control circuitry 104 is identical to the X servo circuitry, motor,and X tool control circuitry, only one such circuitry will be discussedhereinafter. However, it should be recognized that in order to achieveat least two degrees of freedom of the workpiece relative to the too],each of the systems must be duplicated.

X-Y Stage 11 A portion of the XY stage II is illustrated schematicallyin FIG. 3, the stage comprising upper and lower platforms 11A and 118respectively, the lower platform IIB being connected in a conventionalmanner to a Roh'lix (trademark of Barry Wright Corporation) 14 whichtraverses in the direction of the arrow 14A due to the rotation of theshaft 128 of the motor 12. The stage A is connected, in a like manner,to the motor 13, which is carried by the stage or platform HE, andeffects the motion of the stage IIA relative to the stage "B into andout of the paper as shown by the tail of the arrow 118'. It should berecognized that the XY stage 11 may be of any conventional form as longas it may be driven in some manner by the motors l2 and I3 respectively,and mechanically designed to provide sufficient servo response (i.e.taking into ac count such factors as stiffness, backlash, lead, inertia,resonance. etc.) i

In the first embodiment of the tool 10, as shown in FIG. 4, very minutepositioning of this tool may be obtained by a step and repeatmicropositioning table [4 illustrated schematically in FIG. I, and morecompletely in plan in FIG. 4. In the illustrated embodiment, the table14 is shown housing a drill chuck or the like 18 with a drill 18Adepending therefrom, it being desired to locate the drill precisely withreference to the workpiece W mounted on the X-Y stage II. To this end,the micropositioning table 14 includes a frame 14A which may beconnected to conventional apparatus for moving the member bothvertically, so that the drill comes into contact with the workpiece W,and

horizontally, either manually or under preprogrammed automaticpositioning. However, inasmuch as this does not form part of the presentinvention, but is merely an example of the type of tool and thepositioning thereof which may be utilized in accordance with theinvention, the micropositioning table 14 will only be described in minordetail, a more complete description being set forth in Volume 12, No.ll, April l970 pages I958, I959 of the IBM Technical DisclosureBulletin. In the illustration of FIG. 4, the chuck 18 is mounted in thetable 2 which is suspended from the frame MA as by identical leafsprings I, mounted at right angles to each other on the corners of thetable 2. The springs have a high spring rate along the Z axis and lesserbut equal spring rates along the X and Y axis of the table 2. The springrates along the X and Y axis imparting repeatable positioning capabilityfor the table. Thus the spring rates provide a deflection proportionalto the X and Y forces imparted by force actuators or drive means 4 andS.

The X servo circuitry, which is used as the example hereinafter, servesto take the desired position of the workpiece W, compare it to itsactual position, and then drive the stage until the workpiece fallswithin a reasonable tolerance of where it is supposed to be, hereinafterreferred to as site," and thereafter provide any difference between theactual and desired position signal to the tool control circuitry 75 todrive the tool to effect a precise positioning of the tool relative tothe workpiece. To this end, and referring first to FIG. 2C, the stagedrive means 21 includes the motor 12, a tachometer 22 which may beintegral with the motor 12 and which applies the feedback signal 12A toa tachometer buffer electronics 22A, which in turn applies a feedbacksignal through lead 128 to a motor drive amplifier 23, and then througha booster amplifier 24, which closes its loop by providing an input 24Ato the stage drive motor 12. The stage drive means 21 may be referred toas a servo drive amplifier with tachometer damping feedback, such asystem being shown in US. Pat. No. 2,674,708 to H. L. Husted. The motordrive amplifier 23 may be a standard off-the-shelf Inland Motor, ModelNo. EMl80l and purchased directly from Inland Motor Corporation ofRadford, Va. The booster amplifier 24 is also standard and may beutilized, in accordance with Inland Motors instructions for boosting thepower output of the EM 1801 from 25 to, for example, 200 watts. Thedrive stage motor 12 may be a standard servo motor such as an InlandMotors, Model No. NT 2909A, the tachometer 22 which, while beingseparate, may be mounted integrally with the motor and connecteddirectly to the drive shaft which leads to the stage II. A typicaltachometer is the Inland Motor Model No. TG-13l8C.

The tachometer 22 feedback signal is applied to the tachometer buffer22A which is an operational amplifier adapted to filter out the noisegenerated by the tachometer (brush commutation in the tachometer) whichgives a velocity (relative to voltage) feedback signal through line 12Bto the motor drive amplifier 23 causing the motor drive amplifier toeither increase or decrease its signal to the booster amplifierdepending upon the positional information received, and discussedhereinafter, to the motor drive amplifier 23 from its first input at23A. The tachometer buffer 22A is a purchased part and may be, forexample, a Philbrick operational amplifier such as their part numberl0l6.

In order to stop the positional signal on line 23A to the motor driveamplifier when certain conditions exist, and therefore to stop the motor12, a stop switch 25 may serve to open the positional signal informationgiven to the motor drive amplifier 23. To this end, the stop switch, ineffect, is an on-off switch, and may be a dual single pole single throwsolid state switch such as the Dickson DAS 2l37-1, although a relay withappropriate contacts may be utilized in lieu thereof. The operation ofthe stop switch in conjunction with the stop logic, is describedhereinafter.

Coupled to the stop switch is a band reject filter 26, to which thepositional information may be applied as at input 26A, the positionsignal passing through the band reject filter and the stop switch 25 tothe motor drive amplifier 23. The band reject filter 26 is a notchfilter which attenuates the amplitude of the drive signal at theresonant frequency of the X-Y stage. For example, if the table resonatesat 50 cycles, the band reject filter is tuned or otherwise disignatedfor that particular frequency to minimize the amplitude at thatfrequency. The concept of a band reject filter for this purpose is notnew in the art, for example see US. Pat No. 3,660,744 issued on May 2,1973 to Plummer.

Although the desired position input may be fed to the X servo circuitry20 in either analog or digital form, the preciseness of digital form ispreferred for such input to the circuitry. Accordingly, in order todrive the motor drive amplifier and thus the servo motor 12, means areprovided for converting the digital information to analog information ora voltage level corresponding to the difference between the desiredposition and actual position, the digital difference preferably beingexpressed in the binary coded decimal format for ease of humanreadability, although it should be recognized that conventional digitalbinary numbers may be utilized to control the system. To this end andreferring to FIG. 28, a digital to analog converter 27 receives adifference signal in binary coded decimal form through inputs 27A, andprovides an analog output along line 27B to a function generator 28, inthe present instance a square root function generator. The functiongenerator 28 differentiates the difference positional information comingin from the digital to analog converter 27 from output 278 which is avoltage level indicative of the difference in distance that the systemmust move to arrive at a predetermined position relative to some fixedreference. Inasmuch as the function generator is receiving an inputvoltge that corresponds to the ad dress differential between the desiredposition and the actual position of the stage at any instance of time,since the first derivative of a position with respect to time isvelocity, and inasmuch as it is desirable to maintain a uniformacceleration or deacceleration of the stage by way of the servo motor12, the square root function is utilized in the generator 19 so as tomake velocity dependent upon distance as opposed to being dependent upontime. For an example of a positioning system utilizing a functiongenerator for this purpose, see U.S. Pat. No. 3,241,0[5 issued on Mar.15, 1966 to Allen. The function generator may be an off-the-shelf modulesuch as the model 4095/l5 made by Burr- Brown Corporation. The binarycoded decimal number to analog converter 27 may be one of severalpurchasable converters such as the Cycon Inc. converter Mod.

CY2735, made by Cycon Corporation of Sunnyvale, Calif.

The square root function generator performs acceptably in theimplemented embodiment because inherent time constants in the hardwaretend to attenuate the instantaneous changes in acceleration at thestart, middle and end of the stepping increment. This potential impulsefunction has been treated extensively in cam design literature, and iscommonly called jerk or pulse to define the instantaneous rate of changeof acceleration. A suitable treatment of pulse is in CAMS Design,Dynamics, and Accuracy, Harold A, Rothbart, John Wiley and Sons, lnc.,New York, 1956, Chapter 2. The treatment deals with the parabolicfunction which the square root function theoretically generates and themore desirable cycloidal function.

The output of the function generator is through a gain control amplifier29 which serves to limit the voltage to the band reject filter 26 andultimately the motor drive amplifier 23. The gain control 29 may be setat some selected level, as by limiting the gain of the amplifier byinput 29A from the control 100, as when the stage 11 approaches itsfinal position or site. There are three separate conditions which canapply to a stop signal to the stop switch and cause the motor drive amplifier 23 to prevent further driving of the motor 12, these being: (l)position stop, that is when the table reaches the position for which itwas intended i.e. within the site; (2) a positive limit stop; and (3) anegative limit stop.

To this end and referring to FIG. 2C, a stop logic circuit isillustrated as having four inputs, a first input 30A from a latch stopcircuit 35, hereinafter described, and which indicates that the stage 11has reached its desired position or site, a second input 308 whichindicates that the stage 11 has travelled beyond its limit in thepositive direction, and a negative limit input 30C which indicates thatthe stage has moved in the nega tive direction into its limit switch, Afourth input, 30D, is applied to the stop logic circuitry to reverse thepolarity to allow the stop logic to come of the limit switches. Theoutput of the stop logic 30 is applied through line 25A as an input tothe stop switch 25.

Thus the stop logic 30 acts as an interface between the various stopcommand sources and the stop switch 25, which when open, stops anysignal from being ap plied to the motor drive amplifier 23 and stops themotor 12. To this end, and referring now to FIG. 5, three TTL modulesG1, G2 and G3 make up the gating network for controlling the on-offstate of the stop switch 25 and therefore the motor 12. G1 may be a type7404 hex inverter, that is it has six inputs and six outputs. Gl drivesG2 which is a type 7400 quad, two input NAND gate. The outputs of the G2module drive and G3 module which may be a type 7420 dual, four inputNAND gate.

The outputs from module G1, the hex inverter, are always opposite theircorresponding inputs. The operation of the NAND logic blocks, G2 and G3can be explained by saying that the only way to get a logic low outputis to have all inputs at a logic high. All logic lows or any high, lowcombination on the NAND inputs results in a logic high out,

Referring now to the three ways in which the stop logic 30 effects astop or opens switch 25, the various stop signals 30A30C will be treatedserially.

]. Position Stop When the table or stage ll reaches near the position orsite intended, a logic high is applied to input 30A to the stop logic30. This input emanates from the latch stop circuit 35 which will bediscussed hereinafter. The logic high from input 30A is then applied toan inverter GlA which produces a low at one of the inputs of NAND gateG3. Although NAND gate G3 has three other inputs, regardless of theirlevels, a low at the first input to G3 (ie the output of 01A) demandsthat the outut of G3 on line 25A, is high. (Any low applied to the inputof a NAND gate produces a high ouput).

2. Positive limit stop If for any reason the stage arrives at the end ofits travel in the positive direction, a positive limit stop for examplea limit switch (not shown) will stop the motor. When the table is notagainst the positive limit stop, the input at 308 is a logic high, whichis provided by pullon resistor R1, typical values of which are given inFIG. 5. lfa positive limit is reached, a logic low is applied toinverter GlB. GlB, in turn places a high on one input ofGZA. lnasmuch asthe motor 12 was moving in a positive direction, the sign input at 30Clis at a logic high which is applied to the second input of NAND gateG2A. The output, therefore, of NAND gate G2A goes low which effects ahigh or an up output along line 25A from NAND gate G3.

3. Negative limit stop Should the table reach a negative limit, that isstrike a limit switch (not shown) indicating that it had travelled toofar in the negative direction, the high level on the input to GlD isreplaced by a low, the high level being provided initially by pullupresistor R2. The output of inverter GlD, which is high therefor, isapplied as an input to NAND gate G28, Since the motor 12 was moving in anegative direction (in order to hit the negative stop) the sign inputalong input 30Cl is at a logic low. Because the low is applied toinverter GlC, the upper input to NAND gate GZB is a logic high causingthe NAND gate G2Bs output or input to NAND gate G3 to be a low,effecting a logic high output on line 25A from NAND gate G3.

Simply stated the limit stops, above described, inhibit motion when alimit switch is actuated in the direction toward the limit switchAlternately, the motion in the direction which would back the table offthe actuated switch is not inhibited.

Thus all three stopping modes effect a high output at G3. Such a highoutput along line 25A causes the stop switch 25 to open therebypreventing further driving of the motor 12.

in order to provide the digital to analog converter 27, via its inputbus 27A with a binary coded decimal difference between the desiredposition and actual position of the stage or workpiece, comparing means31 is provided, in the present instance the comparing means comprising adigital error generator. To this end, and referring first to FIGv 2B.the digital error generator includes a first or primary input 31A towhich is fed the BCD address corresponding to the desired position ofthe workpiece or stage relative to some fixed reference. A second inputbus 318 feeds a binary coded decimal number corresponding, at any oneperiod of time, to the actual position ofthe workpiece or stage from aposition indicating means 40, which is described hereinafter. Althoughthe digital error generator may be of conventional structure, theparticular structure utilized is best illustrated in FIGS. 6-9.

The digital error generator takes the form of a binary coded decimalsubtractor. To subtract two numbers of the base It), the subtrahend is9s complemented (each digit is subtracted from 9) and then added to theminuend. In this system, if the sum has a carry, the carry is broughtaround and added to the least significant digit (LSD) of the sum. Thistechnique is commonly referred to as end around carry." The sum is thenequal to the dividend of the original subtraction. If the sum does nothave a carry, then the sum is 95 complemented. This means that thedividend is negative. Thus the digital representation of the stageposition which is referred to in the drawings as input B, correspondingto the bus 318, is subtracted from the desired position, referred to inthe drawings as input A corresponding to the signal from bus 31A, usingthe 9s complement method. The dividend is converted by way of thedigital to analog converter 27 and, in the manner heretofore described,fed to the motor drive amplifier 23.

It should be noted that the subtraction could be made using a 10'scomplement method (each number is subtracted from 9 and then l is addedto the result). This would not require the carry to be added to the sum,but does require additional logic necessary to add I when the number iscomplemented. As in the 9s complement method, if there is not carry thesum is then lOs complemented and considered negative. If there is acarry, it is just ignored. However, because of the additional circuitryrequired for the lOs complement method, the 9s complement method may bepreferable, and as illustrated in FIGS. 6-9.

Turning now to FIG. 6, the overall scheme of the comparing means ordigital error generator 31 is illustrated therein. In the embodimentshown, the input A corresponds to the address of the desired position ofthe table, while the input B corresponds to the address in binary codeddecimal numbers of the actual position of the stage or table. Asillustrated, each pair of inputs is coupled to a BCD adder 32A, 328through 32N, each of the adders having an output therefrom coupled togated 9s complement circuitry 33A, 33B, 33N. The dividend or output fromeach of the gated 9s complement circuitry blocks is the binary codeddecimal number corresponding to the difference between the ad dress ofthe actual position and the address of the desired position. For ease ofreading, the least significant digit (LSD) to the most significant digit(MSD) has been labelled both at the input and output. As isconventional, each of the binary coded decimal (BCD) adders 32A-32N hasa carry input (Cl) and a carry out put (CO) each preceding carry out (C)being coupled to the next succeeding carry in (Cl) except the last carryout in the binary coded decimal adder 32N, corresponding to the mostsignificant digit MSD, is coupled to an inverter 34 the output of whichis coupled to each of the gated 9s complement 33A-33N and the input ofwhich is coupled to carry in (C1) of the binary coded decimal adder 32A.Additionally an output indicative of the sign, as marked, is taken fromthe carry out of adder 32N (MSD). As illustrated each of the B inputs,before being applied as an input to the binary coded decimal adderpasses a 9s complementing circuit designated 31BI, 31B2, and 31BN. Inoperation,

input A is the minuend while word or input B is the sub trahend, thesubtrahend being complemented by the 9s complement circuit and thenadded in the adder to word A. In the instance where there is no carry,the sum of A and B (A B) is complemented. On the other hand if there isa carry, the sum is increased by l but not complemented. This occurs inthe gated 9's complemented circuitry.

Each of the 9's complement circuits includes an inverter 31C, andexclusive OR logic gate 31D and a NOR gate 31E, the inverter 31Creceiving the least significant bit of the group of bits 2 2 As shown,the next most significant bit, 2, is coupled directly to the output,while it is also provided as an input along with 2 bit to the exclusiveOR gate 310 to provide an output 2 The NOR gate 31E receives inputs fromboth the 2 and 2 as well as the 2 bit, the gates acting in aconventional manner to apply an output therefrom which acts as the inputto its associated binary coded decimal adder. To aid in an understandingof the design philosophy of the 9s complement and the gated 9scomplement circuits, the following table with notes is supplied.

No. Compl. BCD COMPL 2 2 2 2 2 2 2 2 0 9 0 0 (1 0 l 0 0 I l 8 0 O O l l0 0 0 2 7 O 0 l 0 O I l l 3 6 0 0 l l 0 I i 0 4 5 O l O 0 0 l O l 5 4 0i 0 I 0 l 0 0 6 3 0 I I 0 0 (I l 1 7 2 0 l l l 0 0 l 0 8 l I 0 0 0 0 0 0l 9 0 i 0 O l 0 (1 0 0 2 is always inverted in complementing 2' is neverinverted in complementing 2 is inverted only when 2 is high. The twoinversion states being 00 0 D1 l exclusive or I0 l I I 0 2* is invertedwhen 2 and 2 are 0 all zeros I NOR any not zero 0 The gated 9scomplement circuitry is illustrated in FIG. 8. The circuitry providesboth complemented and uncomplemented bit status for each digit, thecomplementing being achieved essentially as described above. The properform of the bits is then gated through to the output by enabling one orthe other of the AND gates, for example, 338 or 33C. Thus when the gateinput to the inverter and the AND gates is 0 the number hecomes 9complemented.

FIG. 8 illustrates the complemented digits by C2" and the uncomplementeddigits by G2". The exclusive or 33D essentially performs the samefunction as the gating ANDs 33B and 33C and an inverter needed tocomplement the 2' bit, and is an equivalent circuit. Similarly the 2 bitreduces to a single line as there is never an inversion.

The binary coded decimal adders 32A-32N may be constructed of standardparts as shown in FIG. 9. For example, a pair of conventional binaryadders may be joined as illustrated in the drawing. The inputs to thefirst binary adder 36A are the binary coded decimal number correspondingto the desired position and the binary coded decimal numbercorresponding to the actual position which has been 9 complemented. Thepair of AND gates and OR gate are utilized in conjunction with theoutput of the first binary adder to render a carry out or carry overposition to the next succeeding binary coded decimal adder.

The position indicating means signals the digital error generator 31 asto the position of the stage II, during any particular point in time,and includes an encoder position register 41 which sends a signal alongline 41A of the actual address of the stage in binary coded decimalnumbers. The output 41A is branched into 41Al, and 41A2, the branch 4lA1providing the B input as described relative to FIGS. 6-9, to the digitalerror generator 31. The second branch, 41A2 may or may not be utilizeddepending upon the control system employed. Various control systems willbe discussed here inafter. The encoder position register 41 has a secondoutput preferably to a display of the actual address 42,

which is a conventional display illustrating the position of the tablein its binary coded decimal number form at any one period of time sothat the machine operator, if the machine is operating under manualcontrol, may determine the exact position of the stagev The encoderposition register 41 receives an input from an elec tronic reading headand light source 43 which is optically coupled to a movable scale 44mounted on the stage, the reading head and light source continuouslymonitoring the exact position of the stage relative to a fixedreference.

The position indicator means may be purchased as an off-the-shelf item,and may be one of several types. The system employed, however, is theTeletrak an absolute digital readout system, supplied by Whitewell Electronic Developments Ltd. at l 1 Watt Road, Hillington, Glassgow.

The basic requirement of numerical position measuring systems is theconversion of the analog to digital values because the information inthis particular format is better suited for data processing and providesa much higher resolution than may be obtained from practical analogdevices. Additionally, as with the use of the display of the actualaddress, the information may be easily converted to a numerical displayfor human read ability.

There are several types of analog to digital converters when absolute oractual position information is necessary, one of the principal types hasbeen the rotary encoder or digitizer. One version of the device (rotaryencoder) translates angular position into digital values by the use of amechanically rotated shaft with pickup brushes in contact with a statorin the form of a coded disk. The disk includes a plurality of concentricrings which are divided into equal conducting and nonconducting sectorsin either a binary or a binary coded decimal sequence. Because ofmechanical imperfections, and the small size of pickup brushes,positional errors arise from the use of the coded disk type encoderdigitizer. With the disk type encoder or digitizer, therefor, it isnecessary to take special precautions to avoid am biguity in the readingfrom the positioner.

A preferred type of position indicating means is an optical defractiongrating. When two gratings are suitably arranged, interference patterns(MOIRE fringes) are produced of a wavelength equivalent to the pitch ofthe gratings and of dimensions suitable for use in metrology. Thegratings may be manufactured in a con ventional manner and themaintenance of a uniform gap between gratings and the relative parallelmovement are not excessive and are well within the limits of normalaccurate machine practice.

A more complete writeup of the "Teletrak" system may be found in theAugust 1969 issue of Automation (an English publication).

The digital error generator continuously provides a flow of informationto the digital to analog converter 27 and thus to the drive motor 12,the information being in the form ofa signal indicative of thedifference between the actual position of the stage or table and thedesired position. At some time when the difference between the desiredposition and the actual position reaches a finite or small difference,it is necessary to transmit a signal to the stop logic circuitry, andthus the stop switch 25, to open the flow of information to the motordrive amplifier 23 and thus stop the motor 12. To this end, the digitalerror generator 31 also transmits a signal to a position stop generator38 from and along line 388. In its simplest form, the position stopgenerator 38 is a comparator, which takes an input comprising thedifference between the actual and desired table or stage position, andcompares it with a preset predetermined value such that when thedifference signal is equal to or less than the preset difference, anoutput signal is generated along line 38A to the latch stop circuitry 35so as to transmit a signal along line 35B, and 30A to the stop logic 30(see FIG. 5) thereby opening the circuit and stopping all positionalinforma tion from being transmitted to the motor drive amplifier 23.

It should be recognized that other methods may be employed to set theposition for stopping the drive signal from being transmitted to themotor drive amplifier 23, for example, a signal input from the digitalerror generator indicating a sign changeover from, for example, plus tominus may be utilized in conjunction with the stop logic 30 to effect astopping of the transmitted signal to the motor drive amplifier.Additionally, a differential voltage indicative of velocity may be takenfrom the tachometer buffer 22A and directed to the position stopgenerator 38 such that when the voltage generated by the tachometerreaches a predetermined lower limit or changes direction, the comparatorreceives and compares it with a predetermined voltage causing thecomparator to change state and provide an output which effects a stopsignal through the latch stop circuit 35, via output line 38A, to thestop logic 30.

In order to provide a latching condition to the stop logic 30 undercertain predetermined conditions, a latch stop circuit is provided. Thepurpose of the latch stop circuit 35 is to generate a continuous stopsignal to the stop logic 30 when one of the stop conditions, such as adigital error less than some preset, predetermined value, generated bythe position stop generator 38, even after such stop condition may nolonger exist. Once such stop condition is latched it is then necessaryto reestablish a new go signal 35A (see FIG. 28) to again actuate orenergize the motor 12. This condition is necessary inasmuch as it ispossible that the stop con dition which generates the stop signal maycease to exist even though the stop si nal is still desired. Forexample, if the comparison sfgnal in this position stop generators 38fixed input is very small it is possible to coast through the siteeffecting an opposite sign difference signal before stopping. Without alatch the stop signal would cease and the drive motor 12 would againdrive the table in the opposite direction precipitating a hunting modeposition servo which, as set forth in the objects, is an objective ofthis invention to eliminate. The latch is also designed to provide forstopping of the motor 12 prior to a generated stop signal by droppingthe go signal 35A which enables manual or automatic stopping due to someemergency or desire for a premature stop.

Referring now to FIG. 10, the latch circuit of the latch stop 35 isillustrated therein. A go signal on input 35A received from theautomatic/manual switching gate 50 and thus the control 100, serves toset the flip flop 35E which raises the level of the stop/no stop signal3513, the signal 358 being utilized as an output to the input 30A ofstop logic 30 (see FIGS. 10, 2B and 2C). Go signal 35A, also serves toclear the flip-flop 35F whose output 35D feeds back to NAND gate 35G andAND gate 35H which control the flip-flop 35E. As shown, a reset output35C, which is the opposite of the signal on 35B, is fed back to theautomatic/manual switching gate 50 to indicate the latched and unlatchedcondition of the stop 35 and thus the motor 12.

As has already been explained when the stage has reached its approximateposition, within a predetermined site, the position stop generatortransmits a signal which sets the stop logic 39 and thus opens the stopswitch 25. In accordance with another feature of the present invention,prior to starting the circuits which control the tool, it is necessaryto ensure that all motion of the stage has ceased. To this end, a ringout detection circuit 45 is provided intermediate the tool circuitry(each of the X and Y tool circuitry 75 and 103) and the encoder positionregister 41 (see FIG. 2B). The ring out detection circuit is bestillustrated in FIG. 12 wherein an input signal is received from theencoder position register from line 45A, the input signal comprising, inthe present instance, the least significant digit. The least significantdigit input from line 45A is coupled to a first and second row ofinverters, the first row including inverters 46A, 46B, and the secondrow including inverters 47A, 47B, 47C, and 47D. In the first row,intermediate inverters 46A and 46B is a capacitor 46C which is connectedto ground, while intermediate inverters 47B and 47C in the second row isa second capacitor 47E connected between that row and ground. Each ofthe rows terminates in a first input 1 to an AND gate 48 and NOR gate 49respectively, the second inputs being cross-coupled so that the outputof inverter 47D comprises the first input of NOR gate 49 and the secondinput of AND gate 48, while the output of inverter 468 comprises thefirst input of AND gate 48 and the second input of NOR gate 49. Theoutputs 48A and 49A are connected from their respective AND and NORgates to an OR gate 50' which has an output therefrom 45B indicative ofa stop ringing condition. To aid in understanding of the operation ofthe circuit, the input signal from line 45A is designated (M) and theoutput from inverter 46B is labelled (N), while the output from inverter47D is labelled (P), therfore both (N) and (P) are applied to both theAND and NOR gates.

The circuit illustrated in FIG. 12 is used to detect when the last bitof the position encoder has stopped changing for a fixed period of time,for example the time A,. As may be seen, (N) follows (M) when (M) goesfrom a low to high, but has a certain time delay A, due to the action ofthe capacitor 46C, when (M) goes from high to low. This occurs when TTLlogic is used. Alternatively (P) follows (M) when (M) goes from high tolow but has a delay time, A when (M) goes from low to high. As long as Mchanges state at a rate less than A then (N) cannot equal (P). However,when (M) changes state at a rate greater than A,, then (N P).Accordingly, the output will be high when the input remains in eitherstate for longer than A,. Thus by altering the size of capacitors 46Cand 47E, for example, the ring out time may be adjusted so that when theleast significant digit (LSD) from the encoder position register 41changes at a predetermined slow rate due to the dumping characteristicsof the stage, the output signal along output line 458 of the ring outdetection circuit 45 will go high indicating a stop ringing" condition.

The automatic manual switching gate 50 is nothing more than a pluralityof switches to receive an input from either a manual control orautomatic control circuitry 100, discussed hereinafter. To this end, andturning now to FIG. 11, wherein a portion of an auto/- manual switchinggate circuit is illustrated, the circuit 50 includes a signal input 50Awhich emanates either from the automatic portion of the control circuitor from the manual control circuit, the signal being utilized to closeeither the automatic NAND bus circuit 51, or the manual NAND bus circuit52. Each of the blocks 51 and 52 illustrated in the drawing may becomprised of, for example, a flip-flop or relay having an output whichsets a plurality of switches to permit a flow of information from eitherthe automatic input (Al-A4) or the manual input (Ml-M4) to variousparts, heretofore described, of the system. If each of the circuits 51and 52 require an up level input, (digitized 1) an up input to the NANDbus 51 will affect switch closure and permit the inputs Al through A4 tobe provided to the NAND gates 53A, 53B, 53C, 53N, while a logic highinput, when applied to the inverter 54 will cause an invert of the logichigh giving a logic low keeping the buss 52 open so that manual input ofthe system is not possible. Alternatively, a logic low applied to theinverter 54 will cause such an inversion in inverter 54 closing theswitches in NAND bus circuit 52 and allowing the manual input of signals(Ml-M4) to the NAND gates 53A-53N. Thus the NAND gates 53A-53N mayreceive an input from either the automatic section of the controlcircuitry 100 or the manual section depending upon the activation of anyparticular bus 51 or 52.

As shown in FIG. 11, various of the outputs of the NAND gates arelabelled, for example NAND gate 53A provides an input to gain control 29to allow presetting of the gain controls gain so as to reduce thevelocity of the motor 12 when desired. Additionally, the NAND gate 538which receives an input A3 or M3, serves to transmit an output signal 54which branches into an output 54A to a gate (described hereinafter) andto the line 35A to effect an unlatching of the latch stop 35 (see FIG.2A and 2B). The NAND gate 53C receives an input from either the signalM2 or A2 depending upon which NAND bus circuit 51 or 52 is energized,the signals A2 and M2 serving to reset the encoder position register 41,as by a signal 55 which inputs the encoder position register 41 with thehigh digits and bits to reset the encoder. Inputs M1 and A1 feed aplurality of NAND gates 53N through one of their respective NAND buscircuits 51 and 52 when energized, provid ing an output along bus 31A ofthe desired stage position relative to some fixed reference.

There are, of course, inputs as well as other outputs that may be usedin accordance with the embodiment shown in FIG. 11, for example an inputregister may include a plurality of NAND gates such as 53A-53N toreceive various input information. such as an output 458] of the ringout detection circuit 45 indicating that the stage has stopped; theoutput of the encoder position register 41 along line 4IA2 indicatingthe actual position of the stage; and input from latch stop 35 alongline 35C (see FIG. to indicate the control circuit 100 the condition ofthe latch stop and thus the stop logic 30. The output of the NAND gatsreceiving such inputs would be split into NAND bus circuits similar tothose described heretofore relative to circuits SI and 52, so that whenenergized, the bus would provide an output to the respective one of thecontrols, either automatic or manual that had been energized. Otheroutputs are taken from the automatic manual switching gate to, forexample, displays 57 and 58. For example, display 57 may be abinary-coded-decimal number display of the desired address while thedisplay 58 illustrates the binary-coded-decimal number corresponding tothe actual address.

After the latch stop has been set cutting off the motor 12 and the ringout detection circuitry 45 transmits a signal along line 458 indicatingthat the stage has set tled out, means are provided for energizing thetool 10 to effect a deflection AND the tool an amount equal to theremaining difference between the desired position and the actualposition of the workpiece so as to enable precise positioning of thetool relative to the workpiece. To this end, and referring first to FIG.28, a gate (fine mode) 76, which is part of the tool control circuit 75(see FIG. 1) is enabled allowing at least the low order bits indicativeof the difference between the actual and the desired position, to beapplied to a digital converter 80, which converts, in the presentinstance, the digital information and applies the same to a tool driver90, for effecting motion of the tool 10. Turning now to FIG. 13, thegate 76 includes a pair of AND gates 77 and 78 and a gating bus 79, theand gate 77 receiving a first enabling input along line 54A from theoutput 54 of the auto/manual switching gate 50 (see FIGS. 2A, 2B and11). The second input to the AND gate is 3581 from the latch stop 35,the signal from the latch stop being up when the condition is determinedthat the difference between the desired and the actual address of thestage or table is within the site. The third input to AND gate 77 isfrom the ring out detection circuit 45, notably input 4582, it beingrequired as is con ventional with AND gating circuitry, that all inputsbe up before an output 77A is transmitted therefrom. The output 77A ofAND gate 77 applies a first input 77A] to AND gate 78. The second inputto AND gate 78 is transmitted from a fine operation complete circuit 97,which includes an AND gate 98 and an inverter 99. The only time that theinverter 99 receives an up signal on its input 99A so as to apply a downsignal at its output 998 is when the input is up and that occurs onlywhen the actual address B equals the command address A. (A' A B orDesired Position minus Actual Position). Thus when there is a differencebetween the acutal and desired address the signal which, along with theup signal on input 77A1 of the AND gate 78 enables the AND gatepermitting it to transmit a signal 78A along line 78A to enable a gatingbus 79. The gating bus described previously in reference to FIG. 11allows transmission of data when the enable line, in this case 78A, isup. The gating bus 79 receives the output signal (A') from the digitalerror generator 31, which input is designated 31C for purposes ofidentification. The difference signal A enters into digital converter 80(FIG. 13) via bus 81, the least significant digit which is comprised, asis conventional, of four input lines representative of the bits 2" 2",and represented by a box la belled LSD, the next most significant digit,i.e. the least significant digit LSD+l also representing four linesagain repesentative of the bits 2 2 and the least significant digit LSD2 which is also comprised of four wires 22 bits. The input asillustrated is inherently a multiple, the magnitude of the leastsignificant bits being times 1, for the next most significant bits ofcourse times 10, and for the most significant bits times 100, the outputbeing in digital form (BCK). The input A from bus 81 enters into anadder or BCD to binary converter 82 which places the output in a binaryform to a function multiplier 83 which multiplies the binary output ofthe adder to obtain the proper number of steps of, for example, astepping motor per digit. [It

should be noted that the adders and function multipliers are standardoff the shelf items and comprise conventional adder and functionmultiplier circuitry] For example the adder 82 may be a DigitalEquipment Corporation (DEC), Maynard, Mass. M230 as shown in DECs LogicHandbook (1972) at pages 62 and 63. The function multiplier may be forexample, a Texas Instrument 4 bit by 4 bit parallel binary multipliersuch as illustrated on pages 496 and 497 of Texas Instru ments TTL DataBook for Design Engineers, 1973.

The output of the function multiplier 83 is applied to driver circuitry(see FIG. 2B, and FIG. 13) to apply the correct number of steps to thetool 10 to bring the tool into the proper position relative to theworkpiece. To this end and referring once again to FIG. 13, the driver90 comprises a comparator 91, to which is applied the output of thefunction multiplier 82 along out put bus 82A, and to which is appliedthe output from a pulse counter 92. A square wave oscillator, some timesreferred to as a pulser or clock 92 applies a pulse to the pulse counter92 as through output 925, the output 92A of the pulse counter 92 beingapplied to the comparator 91. The pulse counter either adds or subtractseach pulse from its prior total depending upon whether the differenceaddress A is greater than or less than the actual address B, asindicated by inputs 91A or 918. (For information purposes, the lineshave been designated A greater tha B and A less than B). The outputs ofthe comparator along lines 91A and 91B is also applied as well as adriver 94 to control the direc tion of the driving means which providesthe voltage and current necessary for driving, for example, the steppingmotor clockwise or counterclockwise associ ated with the tool 10, in theillustrated instance stepping motor 5 (see FIGS. 13 and 4).

The comparator has a tertiary output along line 91C which is applied tothe fine mode complete circuitry 97, and more specifically to theinverter 99, the output 91C branching into an input 99A to the inverterand to a second branch line 99B which is applied, as noted, to theautomatic/manual switching gate 50. As heretofore described, when theactual address equals the desired address the output of the comparatoralong line 91C goes up causing the inverter to go down providing aninput to AND gate 98 and to 78 disabling those gates.

For ease of construction, the modules in the driver may be off-the-shelfitems such as:

Comparator 91, DEC M l68, pp. 48 and 49 of the DEC Logic Handbook(Supra) Pulse Counter 92, DEC M236, pp. 66, 67 of DEC Logic Handbook(Supra) Pulser 93, DEC M401, pp. 80, 8] of DEC Logic Handbook (Supra)Driver 94, DEC K202, pp. 166, l67 of DEC Logic Handbook (Supra) Thecontrol unit 100 (FIG. 1) may include an automatic and manual control toeffect the necessary or desired movement of the stage 11 and then thetool as heretofore described. The automatic control may include, forexample, a punched or magnetic tape input as well as card eithermagnetic or punched which automatically inputs the automatic manualswitching gate 50 with the required digital information of the desiredaddress. In the same manner, the manual input may include a conventionaltypewriter like input to input the system with digitized information asto the desired position of the stage 11, or thumb wheel switches. Atypical example of a tape drive that may be employed is an IBM 5028operator station, or an IBM 2501 card reader while a typical example ofthe manual input sys tem is a Cherry Electrical Products Corp. Series L-lever wheel switch.

It should be recognized that other control systems may be employed. Forexample, and as illustrated in FIG. 15, a process control computer 120,such as the IBM System 7, may be employed as a basic input to theautomatic manual switching gate, the computer having a first output bus121 for inputting the desired address to the switching gate 50, a firstinput bus 122 from the switching gate 50 to indicate, if desired, theactual address of the stage, and a control and status bus 123, whichmay, for example, input the switching gate 50 with information such assetting the gain control through line 29A (see FIGS. 2B and 2C) giving adirect input to gate 76 as through bus 54A (see FIG. 28) etc. It mayalso receive status information from the various parts of the System,for example, polarity reversal conditions as when the stage hits a limitstop, notification from the fine operation complete circuit, etc.

The process control computer 120 may be controlled by a data processingcomputer 124 (such as an IBM System 370, Model I45). The data processingcomputer may be also utilized, in this connection, to provide an output125 to a conventional digital interface 126 for applying work or controlinformation along output bus 127 to, for example, the digital converter80. In this manner the tool, after reaching its desired position, may becontrolled as to its operation on the workpiece W held on the stage 11.To this end, an indication may be taken from the fine operation completecircuit 97 that fine positioning of the tool is complete, such anindication being used to trigger the commencement of actual work on theworkpiece W by the tool. In this connection, it should be noted that themanual address and control entry panel 128, which operates in the samemanner as the manual control described here tofore, acts as a bypasssystem for controlling the position of the tool and permits, entrydirectly into the switching gate 50 as through bus 128A, as well ascontrol information through input line 1288. Manual control of the loopmay be also utilized to apply an input to the manual address and controlentry panel from The manual keyboard 129, in the illustrated embodimentthe keyboard 129 receiving its actual address information as a feedbackloop from the display of the desired address 57 as through line 57A. Thekeyboard may also be used to control the tool by applying a second input129A from the keyboard to the digital interface 126.

It should be recognized that the tool and control circuit therefore maytake many forms, for example, in the preferred embodiment the digitalconverter may take the form of a digital to analog converter 80A (seeFIG. 14) which, through an electrostatic deflection amplifier or driverA may be utilized to control the electrostatic deflection means such asplates 13] and 132 of an electron beam 133 for precise positioning ofthe beam relative to the workpiece W, and also to control the movementof the beam to do work on the workpiece, such as the suitably treatedsilicon wafer. This particular embodiment illustrated in FIG. 14 lendsitself to the use of the embodiment illustrated in FIG. 15 wherein thedata processing computer, after initial positioning of the E-beamrelative to the workpiece in a precise location, is capable of writingcircuitry onto suitably covered chips in a silicon semiconductor waferin a predetermined manner. In the embodiment illus trated in FlG. 14,the fine operation complete circuit 97A may be substantially the same asthat disclosed in FIG. 13, the electrostatic deflection amp 90A beingnothing more than an operational amplifier, the fine operation completecircuit being actuated when the difference between actual and desiredaddress is equal to Zero, or some very small value.

Thus the apparatus of the present invention provides a means ofaccurately positioning a tool relative to a workpiece in a minimum oftime so as to enable maxi mum thru put of operations of the toolrelative to the workpiece.

Although the invention has been described with a certain degree ofparticularity, it is understood that the present disclosure has beenmade only by way of example and that numerous changes in the details ofconstruction, the combination and arrangement of parts, and the methodof operation may be made without departing from the spirit and the scopeof the invention as hereinafter claimed.

What is claimed is:

1. Apparatus for positioning first and second elements in a precisepredetermined relationship one to the other, said apparatus comprising:positioning means for positioning said first element into apredetermined site, a control means having a signal output indicatingthe desired position of said first element relative to said secondelement from a fixed reference; position indicating means to provide asignal indication of the actual position of said first element at anyone time relative to said reference; comparing means having an outputsignal responsive to the difference between the signal output of saidcontrol means and the signal output of said position indicating means;and position drive means to drive said positioning means in response tosaid difference signal until said first element is within saidpredetermined site; and means for stopping said positioning means uponsaid difference signal reaching a predetermined value such as toindicate that said first element is within said site; gate means, ringout detec-

1. Apparatus for positioning first and second elements in a precisepredetermined relationship one to the other, said apparatus comprising:positioning means for positioning said first element into apredetermined site, a control means having a signal output indicatingthe desired position of said first element relative to said secondelement from a fixed reference; position indicating means to provide asignal indication of the actual position of said first element at anyone time relative to said reference; comparing means having an outputsignal responsive to the difference between the signal output of saidcontrol means and the signal output of said position indicating means;and position drive means to drive said positioning means in response tosaid difference signal until said first element is within saidpredetermined site; and means for stopping said positioning means uponsaid difference signal reaching a predetermined value such as toindicate that said first element is within said site; gate means, ringout detection means operatively coupled to said position indicatingmeans for determining the rate of change of the position of said firstelement, and means connecting said ring out detection means to said gatemeans; said ring out detection means including means for supplying afirst signal input to said gate means upon the rate of change of theposition of said first element arriving at a predetermined value; andsecond positioning means responsive to said difference signal forpositioning said second element in a precise location relative to saidfirst element.
 2. Apparatus in accordance with claim 1 wherein saidmeans for stopping said positioning means includes latch stop means;said latch stop means operative, upon said difference signal reachingsaid predetermined value to latch said drive means to inhibit furtherdriving of said position drive means until said latched condition isremoved; and a second input to said gate means from said latch stopmeans when said latched condition occurs.
 3. Apparatus in accordance wthclaim 2, including second position drive means forming part of saidsecond positioning means for driving said second element; meansconnecting said second positioning means to said gate; at least saidfirst and second inputs to said gate from said ring out detection meansand said latch stop means operative to open said gate and effectapplication of said difference signal to said second position drivemeans.
 4. Apparatus in accordance with claim 3 including means forindicating wHen said second element arrives at said precise locationrelative to said first element, and means for applying said indicationto said gate means to thereby open said gate.
 5. Apparatus in accordancewith claim 1 wherein said first element positioning means comprises amotor, said position drive means comprising a motor drive amplifier andmeans for applying velocity feedback to said amplifier to limit thespeed of said motor, and including gain control means to limit thevelocity of said motor.
 6. Apparatus for positioning a workpiece andtool in a precise location relative to each other by positioning in apredetermined site said workpiece and then positioning the tool in aprecise predetermined position relative to said site, said apparatuscomprising: at least one closed loop servo system connected to saidworkpiece, positioning means for positioning said workpiece in apredetermined site, position indicating means for determining the actualposition of said workpiece relative to a fixed reference, and a signaloutput from said position indicator means indicative of said actualposition of said workpiece at any one time; an error generator, andmeans for providing to said error generator the desired position of saidworkpiece relative to said reference; and means for inputting said errorgenerator with said signal output from said position indicator means,and means in said error generator to produce a difference signaltherefrom indicative of the difference between the actual position ofsaid workpiece and the desired position of said workpiece; and meansresponsive to said difference signal to actuate said positioning meansuntil said difference signal is such that said workpiece is in saidpredetermined site; and first means responsive to said differencesignal, when said workpiece is in said predetermined site to stop saidpositioning means, and second means responsive to said differencesignal, to effect movement of said tool, ring out detection means,responsive to the rate of change of position of said workpiece, andmeans for enabling said second means responsive to an output signal fromsaid ring out detection means indicating that said rate of change ofsaid actual position has reached a predetermined lower value, saidsecond means effecting tool movement until said difference reaches apredetermined second lower value to thereby effect precise positioningof said tool relative to said workpiece.
 7. Apparatus in accordance withclaim 6 including latch stop means for latching said positioning meansin said stopped position.
 8. Apparatus in accordance with claim 7including gating means having a first input from said latch stop meansand a second input from said ring out detection means; said gate meansoperative to allow gating of said difference signal to effect movementof said tool upon coincidence of said latching and ring out detectionsignals.
 9. Apparatus in accordance with claim 7 including stop switchmeans to interrupt said difference signal to said positioning means inresponse to said latch stop means.
 10. Apparatus in accordance withclaim 6 wherein said workpiece is mounted on a stage, and saidpositioning means includes a motor for moving said stage in apredetermined line of movement.
 11. Apparatus in accordance with claim10 wherein said workpiece comprises a semiconductor wafer, and said toolcomprises an electron beam means.